1. Field of the Invention
The present invention relates to a liquid crystal display device, and particularly to an In-Plane Switching (IPS) mode liquid crystal display device.
2. Description of the Related Art
There are two primary types of liquid crystal display devices. One of the two primary devices displays an image by making the major axis (the direction of the axis is referred to as “director”) of a liquid crystal molecule rotate in a plane orthogonal to a substrate. In contrast, the other displays an image by making the major axis of a liquid crystal molecule rotate in a plane parallel to a substrate.
The former is typified by a Twisted Nematic (TN) liquid crystal display device and the latter is typified by an IPS liquid crystal display device.
The IPS liquid crystal display device is characterized in that even when a viewer views an image on a display of the device from directions different in relation to the device, since the viewer views liquid crystal molecules only in a direction along the minor axis of the molecule, the viewer is able to view the image regardless of whether a liquid crystal molecule stands up or lies down in relation to a substrate, achieving viewing angle wider than that observed using a TN liquid crystal display device.
This is why an IPS liquid crystal display device has been used more prevalently than a TN liquid crystal display device in recent years.
For instance, Japanese Patent Application Laid-open No. 6(1994)-202127 (hereinafter, referred to as a first conventional technique) and Japanese Patent Application Laid-open No. 9(1997)-318972 (hereinafter, referred to as a second conventional technique) disclose a technique for increasing the aperture ratio of IPS liquid crystal display device.
The IPS liquid crystal display device disclosed in the first conventional technique has driving means consisting of an active matrix and is characterized in that the most part of a portion, facing a liquid crystal layer, of a signal interconnect line used to transmit an image signal to the active matrix is covered via an insulation layer by a conductor. However, embodiments employed in the first conventional technique never give indication of a configuration to make a transparent electrode shield a signal line from unfavorable electric field leakage.
The IPS liquid crystal display device disclosed in the second conventional technique has a component serving both as a common electrode interconnect line and a black matrix (in other words, a common electrode interconnect line serving also as a black matrix or vice versa), and further, a scanning line and a data line, both of which are completely covered by the common electrode interconnect line serving also as a black matrix. The second conventional technique is intended to make the common electrode interconnect line simultaneously serve as a conductor and a shielding film and never gives indication of a configuration to make a transparent electrode prevent electric field leakage from a scanning line and a data line.
Both the IPS liquid crystal display devices disclosed in those conventional techniques are intended to make the aperture ratio of the device and increase brightness of an image to be displayed.
Since a potential difference exists between the data line and the common electrode and between the scanning line and the common electrode, an unfavorable electric field is generated by the potential difference. When the electric field reaches a display region between a pixel electrode and the common electrode, liquid crystal molecules included in the corresponding region the unfavorable electric field reaches are affected by the unfavorable electric field and then are abnormally aligned. For example, when a white window having a black background appears on a screen, pixels displaying black and corresponding to a data line transmitting an image signal by which corresponding pixels are driven to display white are forced to have a gray scale level, which phenomenon is a display problem called “longitudinal cross-talk.”
To prevent this problem, dummy common electrodes provided on both sides of the data line and the scanning line are made to have a wide width and electric fields from the data line and the scanning line are made to terminate on the dummy common electrodes, i.e., electric field leakage from the data line and the scanning line is prevented, or the data line and the scanning line are covered by electrodes such as a common electrode at a voltage potential that does not affect an image to be displayed.
In order to increase the aperture ratio of liquid crystal display device, as is shown in the latter case, it is preferable to form a common electrode to cover a data line and a scanning line.
However, when employing the conventional technique, since a common electrode is formed of a shielding film, an extent to which light beams are utilized in the device unfavorably decreases.
The reason why a common electrode is formed of a shielding film is that display through around a boundary between pixels is affected by light leaking from the pixel next to the boundary. To prevent the unfavorable phenomenon, typically, a black matrix made from a shielding film is formed around a boundary between pixels. Then, the inventors of the application performed simulations and experiments, and determined an extent to which light beams leaking from the pixel next to the boundary affect display through around the boundary between pixels, leading to the conclusion that when electric field leakage from the scanning line and the data line is completely prevented, an extent to which light beams leaking from the pixel next to the boundary affect display through around the boundary between pixels is small and the black matrix can be removed from the device except for the case where an image to be displayed is required to have high quality.